单原子层Fe/N/C催化剂及其原位谱学研究

Pyrolyzed Fe/N/C is one of the most promising non-precious-metal catalysts for the oxygen reduction reaction (ORR), and supposed to boost the commercialization of proton exchange membrane fuel cells (PEMFC). The rational design of pyrolyzed Fe/N/C depends on the atomically understanding of the nature of the active sites, which has long been debated and not clear. The challenges mainly come from highly heterogeneous structures formed during the pyrolysis process as well as lack of suitable surface probes and spectroscopic technique. In our previous work, a single-atomic-layer Fe/N/C model catalyst based on monolayer graphene has been designed to explore the active sites. The proposed model catalyst serves to simplify the catalyst structures and to simulate the topmost atomic layer of normal Fe/N/C, where ORR is catalyzed. Herein, we propose to utilize the single-atomic-layer structure to develop in-situ spectroscopic technique for Fe/N/C catalyst. Electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS) will be employed to in-situ monitor the ORR by taking advantage of million fold enhance of Raman signal of Fe/N/C catalysts. Attenuated total reflection (ATR) method will be utilized in Fourier transform infrared spectroscopy to accrue spectral details of single-atomic-layer Fe/N/C. Based on model catalyst and in-situ spectroscopic techniques, fundamental research on Fe/N/C catalysts will be carried out in this project, including 1) distinguishing ORR active sites from highly heterogeneous catalysts; 2) exploring the nature of activity factors; 3) detecting reaction intermediates and determining reaction pathway; 4) finding out the reason for the loss of catalytic activity. This project is aimed to provide principles for designing highly efficient Fe/N/C catalysts, and to set up a methodology for atomic-level studies on heterogeneous non-previous-metal catalysts.

热解型Fe/N/C作为最具潜力的非贵金属氧还原催化剂，其综合性能的进一步突破，对于推动低温燃料电池商业化应用具有极其重要的意义。然而，热解型Fe/N/C复杂的非均相结构、缺乏合适的表面表征技术和谱学技术，使得其活性位和催化机理研究止步不前，难以理性地设计催化剂。申请人前期工作中将Fe/N/C催化剂简化至单原子层厚度，构造出催化反应界面，排除体相干扰。本项目将利用单层Fe/N/C模型催化剂的单层二维结构，结合壳层隔绝纳米粒子增强拉曼(SHINERS)，实现单层Fe/N/C拉曼信号的百万级增强和原位观察；结合对表面物种敏感的衰减全反射傅里叶变换红外，实现红外光谱原位观察。在此基础上，厘清热解Fe/N/C催化剂的氧还原反应活性物种，探索影响其活性的本质，揭示氧还原过程和催化剂失效过程。本项目的顺利开展将推动Fe/N/C催化剂结构的合理设计，在研究方法学上将为其他复杂非均相催化剂的研究提供参考。

氧气还原反应(oxygen reduction reaction, ORR)是诸多化学能源转化装置的关键步骤，例如低温燃料电池，化工原料H2O2的制备等等。目前，热解的金属氮碳催化剂、非金属的碳基催化剂在氧气活化反应中均具有优异的应用潜力。本项目在Fe/N/C模型催化剂基础上开展原位谱学研究，探索构效规律，为其进一步提高稳定性和活性提供理论指导。取得主要成果如下：1) 制备出表面结构较为单一的氧掺杂碳催化剂，结合表面碱度和碱量测量，观察到pKa=8.2的吡喃酮结构为催化氧气活化的活性位。同时，结合原位电化学红外推测了氧掺杂碳催化剂的氧气活化的反应路径。2) 利用CVD法制备薄层Fe/N/C催化剂，利用电化学拉曼手册观察到FeN4中心的环呼吸振动峰，研究热解过程对此峰的影响，并观察到此峰位置与ORR活性的正相关关系。3）设计基于三相反应界面且具有可忽略的传质阻抗的电解池，在此基础上观察到Fe/N/C催化剂在三相界面具有与两相界面体系中不同的ORR机理。本项目在ACS Energy Lett，Electrochem Commun，Acta Phys Chim Sin，Electrochem Energy Rev等期刊上发表标注论文4篇，申请中国发明专利一项，（合作）培养毕业硕士2人，多次参加国际/国内会议。